The tree's root and the drag hook's element (ADR-0005 decisions 7, 13, 14).

This component is small on purpose. It is the one element carrying phx-hook="StatifierBlocksDrag", and every drag event in the editor arrives through it: the hook is attached to the canvas root rather than to each block, so adding a block adds no listeners and the hook has one lifetime rather than one per node.

It is also the measurement stage. The 2026-08-29 amendment to decision 7 admits a second hook whose whole job is reading laid-out boxes, and every box it reads is relative to this element - which is why this element carries data-sb-anchor="stage" and why the connector overlay is drawn inside it, in its own untransformed coordinate space. The hook itself rides a child element rather than this one, because an element carries one phx-hook and this one is the drag hook's; ConnectorLayer renders that child and says why.

It is also where decision 14's theming lands. The --sb-* custom properties are declared on this element by the package's stylesheet, so a host that wants a different palette sets them here through the theme assign - a map of property name to value, rendered as an inline style - and needs no stylesheet of its own to do it.